The propensity of carbon nanotubes(CNTs) to self-organize into continuous networks of bundles has direct implications for thermal transport properties of CNT network materials and defines the importance of clear understanding of the mechanisms and scaling laws governing the heat transfer within the primary building blocks of the network structures—close-packed bundles of CNTs. A comprehensive study of the thermal conductivity of CNT bundles is performed with a combination of non-equilibrium molecular dynamics (MD) simulations of heat transfer between adjacent CNTs and the intrinsic conductivity of CNTs in a bundle with a theoreticalanalysis that reveals the connections between the structure and thermal transport properties of CNT bundles. The results of MD simulations of heat transfer in CNT bundles consisting of up to 7 CNTs suggest that, contrary to the widespread notion of strongly reduced conductivity of CNTs in bundles, van der Waals interactions between defect-free well-aligned CNTs in a bundle have negligible effect on the intrinsic conductivity of the CNTs. The simulations of inter-tube heat conduction performed for partially overlapping parallel CNTs indicate that the conductance through the overlap region is proportional to the length of the overlap for CNTs and CNT-CNT overlaps longer than several tens of nm. Based on the predictions of the MD simulations, a mesoscopic-level model is developed and applied for theoreticalanalysis and numerical modeling of heat transfer in bundles consisting of CNTs with infinitely large and finite intrinsic thermal conductivities. The general scaling laws predicting the quadratic dependence of the bundle conductivity on the length of individual CNTs in the case when the thermal transport is controlled by the inter-tube conductance and the independence of the CNT length in another limiting case when the intrinsic conductivity of CNTs plays the dominant role are derived. An application of the scaling laws to bundles of single-walled (10,10) CNTs reveals that the transition from inter-tube-conductance-dominated to intrinsic-conductivity-dominated thermal transport in CNT bundles occurs in a practically important range of CNT length from ∼20 nm to ∼4 μm.

A major challenge for multiple quantum well(MQW)solar cells is to extract sufficient photo-excited carriers to an external circuit through the MQW region under forward bias. The present study reports the effectiveness of compensation doping in the i-region, which includes MQWs, for more efficient transport of both electrons and holes. Unintentional p-type background doping occurs in GaAs by inevitable carbon incorporation during metal-organic vapor phase epitaxy, causing undesirable bending of the band lineup in the i-region of p-on-n devices. By cancelling this out by sulfur compensation doping to obtain a uniform electric field distribution, we achieved much a high carrier collection efficiency (CCE) >90% at the operating bias voltage regardless of the excitation wavelength, compared to < 50% without compensation doping. Consequently, cell performance was greatly improved, in particular showing an enhancement of the fill factor from 0.54 to 0.77, and degradation-free quantum efficiency within the GaAs absorption wavelength range. The photoluminescence(PL) intensity from the MQW increased as the CCE decreased at a large forward bias, and radiative recombination loss was significantly suppressed by compensation doping. Furthermore, time-resolved PL measurements indicated a much higher speed of carrier escape from the wells, showing a quicker PL decay time of 7 ns at 0.6 V, compared to 18–51 ns without compensation doping.

InGaN/GaN multiple quantum well(MQW)light emitting diodes were grown on silicon substrate by metal organic chemical vapor deposition. A different barrier was heavily doped with silicon based on the same structure. Temperature dependent electroluminescence was performed on the devices. The results reveal that heavily doping the barrier distant from the n-type layer with silicon causes two emission peaks. As the doped barrier gets closer to n-type layer, the energy gap between the two peaks becomes narrower. Silicondoped in the barrier is believed to generate p-n junction built-in field from the doped barrier towards p-type layer. This field compensates the piezoelectric field in the well(s) between the doped barrier and p-type layer. It results in higher emission energy of this (these) well(s). When the doped barrier gets closer to the n-type layer, the compensation is less significant.

We have investigated the transient photocurrent of rubrene-doped nematic liquid crystal using a time-of-flight examination. Drift mobilities on the order of 10−6 cm2/V s were obtained for both positive and negative carriers. The minimal dependence of the rubrene concentration on the drift mobility is indicative of the ionic conduction as carrier transport process. The product of the drift mobility and the viscosity obeys the Walden rule, further supporting the ionic carrier transport process.

We propose the enhancement of the photovoltaic absorption in thin-filmsolar cells using densely packed arrays (not obviously regular) of non-absorbing submicron or micron-sized dielectric spheres located on top of the cell. The spheres can decrease reflection forming an effective blooming layer. Simultaneously, they can suppress the transmission through the photovoltaic layer transforming the incident radiation into a set of collimated beams. The focusing of the light inside the photovoltaic layer allows enhanced absorption in it leading to the increase of the photovoltaic current. Every sphere focuses the incident wave separately—this mechanism does not require collective effects or resonances and therefore takes place in a wide spectral range. Since the fabrication of such the coating is easy, our light-trapping structure may be cheaper than previously known light-trapping ones and perhaps even than flat anti-reflecting coatings.

Sulfur hexafluoride (SF6) gas, widely used in high-voltage circuit breakers, has a high global warming potential and hence substitutes are being sought. The use of a mixture of carbon tetrafluoride (CF4) and SF6 is examined here. It is known that this reduces the breakdown voltage at room temperature. However, the electrical breakdown in a circuit breaker after arc interruption occurs in a hot gas environment, with a complicated species composition because of the occurrence of dissociation and other reactions. The likelihood of breakdown depends on the electron interactions with all these species. The critical reduced electric field strength (the field at which breakdown can occur, relative to the number density) of hot SF6/CF4 mixtures corresponding to the dielectric recovery phase of a high voltage circuit breaker is calculated in the temperature range from 300 K to 3500 K. The equilibrium compositions of hot SF6/CF4 mixtures under different mixing fractions were determined based on Gibbs free energy minimization. Full sets of improved cross sections for interactions between electrons and the species present are presented. The critical reduced electric field strength of these mixtures was obtained by balancing electron generation and loss mechanisms. These were evaluated using the electron energy distribution function derived from the Boltzmann transport equation under the two-term approximation. The result indicates that critical electric field strength decreases with increasing heavy-particle temperature from 1500 to 3500 K. Good agreement was found between calculations for pure hot SF6 and pure hot CF4 and experimental results and previous calculations. The addition of CF4 to SF6 was found to increase the critical reduced electric field strength for temperatures above 1500 K, indicating the potential of replacing SF6 by SF6/CF4 mixtures in high-voltage circuit breakers.

Our previous study shows that the curvature of the plasma meniscus causes the beamhalo in the negative ion sources: the negative ions extracted from the periphery of the meniscus are over-focused in the extractor due to the electrostatic lens effect, and consequently become the beamhalo. In this article, the detail physics of the plasma meniscus and beamhalo formation is investigated with two-dimensional particle-in-cell simulation. It is shown that the basic physical parameters such as the H− extraction voltage and the effective electron confinement time significantly affect the formation of the plasma meniscus and the resultant beamhalo since the penetration of electric field for negative ion extraction depends on these physical parameters. Especially, the electron confinement time depends on the characteristic time of electron escape along the magnetic field as well as the characteristic time of electron diffusion across the magnetic field. The plasma meniscus penetrates deeply into the sourceplasma region when the effective electron confinement time is short. In this case, the curvature of the plasma meniscus becomes large, and consequently the fraction of the beamhalo increases.

It is found that in specific Cl2plasma conditions, using a nickel hard mask over calcium barium niobate, CBN (a material particularly difficult to etch) significantly improves both sidewall angles and etching selectivity. This mask hardening is due to the competition between NiCl2growth and etching during the process. For applied bias voltage higher than the Nisputtering threshold and substrate temperatures higher than 200 °C, this competition results in net NiCl2growth which drastically improves the etching selectivity. This mask hardening was successfully used to define an optical waveguide with 73° sidewall angle in a 1 μm-thick CBN layer. This effect can potentially be used for the etching of a very large number of complex oxides that are known to be inert and very difficult to etch.

The plasma produced due to streamers guided by a dielectric tube and a helium jet in atmospheric air is herein studied electrically and optically. Helium streamers are produced inside the dielectric tube of a coaxial dielectric-barrier discharge and, upon exiting the tube, they propagate into the helium jet in air. The axisymmetric velocity field of the neutral helium gas while it penetrates the air is approximated with the PISO algorithm. At the present working conditions, turbulence helium flow is avoided. The system is driven by sinusoidal high voltage of variable amplitude (0–11 kV peak-to-peak) and frequency (5–20 kHz). It is clearly shown that a prerequisite for streamer development is a continuous flow of helium, independently of the sustainment or not of the dielectric-barrier discharge. A parametric study is carried out by scanning the range of the operating parameters of the system and the optimal operational window for the longest propagation path of the streamers in air is determined. For this optimum, the streamer current impulses and the spatiotemporal progress of the streamer UV-visible emission are recorded. The streamer mean propagation velocity is as well measured. The formation of copious reactive emissive species is then considered (in terms of intensity and rotational temperatures), and their evolution along the streamer propagation path is mapped. The main claims of the present work contribute to the better understanding of the physicochemical features of similar systems that are currently applied to various interdisciplinary engineering fields, including biomedicine and material processing.

In Hall thrusters, the potential distribution plays an important role in discharge processes and ion acceleration. This paper presents a 2D potential solver in the Hall thruster instead of the “thermalized potential”, and compares equipotential contours solved by these two methods for different magnetic field conditions. The comparison results reveal that the expected “thermalized potential” works very well when the magnetic field is nearly uniform and electron temperature is constant along the magnetic field lines. However for the case with a highly non-uniform magnetic field or variable electron temperature along the magnetic field lines, the “thermalized potential” is not accurate. In some case with magnetic separatrix inside the thruster channel, the “thermalized potential” model cannot be applied at all. In those cases, a full 2D potential solver must be applied. Overall, this paper shows the limit of applicability of the “thermalized potential” model.

We report the complex dielectric function of InSb for temperatures from 31 to 675 K and energies from 0.74 to 6.42 eV. The spectra were obtained by rotating-compensator spectroscopic ellipsometry on bulk InSb. The critical point (CP) energies were determined using numerically calculated second energy derivatives of the data. At low temperature, the CP structures are blue-shifted and significantly sharpened relative to those seen at high temperature. The temperature dependence of the E1, E1+Δ1, E0′, Δ5cu-Δ5vu (0.35, 0, 0), E0′+Δ0′, Δ5cl-Δ5vu (0.35, 0, 0), E2, E2′, E2+Δ2, E2′+Δ2, E1′, E1′+Δ1′, and E1′+Δ1′+Δ1 CPs was determined by fitting to a phenomenological expression that contains the Bose-Einstein statistical factor or to a linear equation. In particular, temperature dependences of CPs below 100 K and those of the E0′, E0′+Δ0′, E2′, E2+Δ2, E2′+Δ2, E1′+Δ1′, and E1′+Δ1′+Δ1 transitions have not previously been reported.

Hydrocarbon foams are versatile materials extensively used in high energy-density physics (HEDP) experiments. However, little data exist above 100 GPa, where knowledge of the behavior is particularly important for designing, analyzing, and optimizing HEDP experiments. The complex internal structure and properties of foam call for a multi-scale modeling effort validated by experimental data. We present results from experiments, classical molecular dynamics simulations, and mesoscale hydrodynamic modeling of poly(4-methyl-1-pentene) (PMP) foams under strong shock compression. Experiments conducted using the Z-machine at Sandia National Laboratories shock compress ∼0.300 g/cm3 density PMP foams to 185 GPa. Molecular dynamics (MD) simulations model shock compressed PMP foam and elucidate behavior of the heterogeneous foams at high pressures. The MD results show quantitative agreement with the experimental data, while providing additional information about local temperature and dissociation. Three-dimensional nm-scale hydrocode simulations of the foam show internal structure of pore collapse as well as provide detailed information on the foam state behind the shock front. Finally, the experimental and MD results are compared to continuum hydrodynamics simulations to assess a potential equation of statemodel for PMP foams to use in large scale hydrodynamics simulations.

We report on the lattice location of implanted 59Fe in - and -type Si by means of emission channeling. We found clear evidence that the preferred lattice location of Fe changes with the doping of the material. While in -type Si Fe prefers displaced bond-centered (BC) sites for annealing temperatures up to 600 °C, changing to ideal substitutional sites above 700 °C, in -type Si, Fe prefers to be in displaced tetrahedral interstitial positions after all annealing steps. The dominant lattice sites of Fe in -type Si therefore seem to be well characterized for all annealing temperatures by the incorporation of Fe into vacancy-related complexes, either into single vacancies which leads to Fe on ideal substitutional sites, or multiple vacancies, which leads to its incorporation near BC sites. In contrast, in -type Si, the major fraction of Fe is clearly interstitial (near-T or ideal T) for all annealing temperatures. The formation and possible lattice sites of Fe in FeB pairs in -Si are discussed. We also address the relevance of our findings for the understanding of the gettering effects caused by radiation damage or P-diffusion, the latter involving -doped regions.

In this paper, we determine the optical constants and carrier mobilities of Si-doped and Be-doped InAlAs lattice matched to InP. The samples were grown using molecular beam epitaxy and characterized using Hall measurements, variable angle spectroscopic ellipsometry, and room temperature photoluminescence spectroscopy. A Moss-Burstein shift in the fundamental absorption edge was observed in both Si-doped and Be-doped materials. We fitted a multiple-oscillator, critical point model to the dielectric function of the materials extracted using the spectroscopic ellipsometry. The tabulated input parameters of this model allow for accurate calculations of the dielectric function of doped InAlAs to be made, which is useful information for simulating a variety of InP-based optoelectronic devices.

Germanium doping of GaNnanowiresgrown by plasma-assisted molecular beam epitaxy on Si(111) substrates is studied. Time of flight secondary ion mass spectrometry measurements reveal a constant Ge-concentration along the growth axis. A linear relationship between the applied Ge-flux and the resulting ensemble Ge-concentration with a maximum content of is extracted from energy dispersive X-ray spectroscopy measurements and confirmed by a systematic increase of the conductivity with Ge-concentration in single nanowire measurements. Photoluminescence analysis of nanowire ensembles and single nanowires reveals an exciton localization energy of 9.5 meV at the neutral Ge-donor. A Ge-related emission band at energies above 3.475 eV is found that is assigned to a Burstein-Moss shift of the excitonic emission.

CdTe detectors are commonly used for X and γ ray applications. The performance of these detectors is strongly affected by different types of mechanical stress; such as that caused by differential expansion between the semiconductor and its intimate metallic contacts and that caused by applied pressure during the bonding process. The aim of this work was to study the effects of stress on the performance of CdTe detectors. A difference in expansion coefficients induces transverse stress under the metallic contact, while contact pressure induces longitudinal stress. These stresses have been simulated by applying known static pressures. For the longitudinal case, the pressure was applied directly to the metallic contact; while in the transverse case, it was applied to the side. We have studied the effect of longitudinal and transverse stresses on the electrical characteristics including leakage currentmeasurements and γ-ray detection performance. We have also investigated induced defects, their nature, activation energies, cross sections, and concentrations under the applied stress by using photo-induced current transient spectroscopy and thermoelectric effect spectroscopy techniques. The operational stress limit is also given.

The dynamics of optical nonlinearities in bulk GaN crystal are investigated by a modified time-resolved pump-probe system with phase object at 532 nm under picosecond pulse excitation. The sample's optical nonlinearities can be determined by measuring the normalized transmittance with/without an aperture in the far field. The different nonlinear mechanisms in GaN are separated. Experimental results show that the nonlinear response of GaN is a combination of bound-electronic and free-carrier nonlinearities mechanisms. With the theory of two-photon-induced free-carrier nonlinearity, the related optical nonlinear parameters, including free-carrier nonlinearities and free-carrier life time which are difficult to be determined, are obtained.

Using an indigenously built r.f. magnetron sputtering system, several single layer Ti and Nifilms have been deposited at varying deposition conditions. All the samples have been characterized by Grazing Incidence X-rayReflectivity (GIXR) and Atomic Force Microscopy to estimate their thickness, density, and roughness and a power law dependence of the surface roughness on the film thickness has been established. Subsequently, at optimized deposition condition of Ti and Ni, four Ni/Ti multilayers of 11-layer, 21-layer, 31-layer, and 51-layer having different bilayer thickness have been deposited. The multilayer samples have been characterized by GIXR and neutronreflectivity measurements and the experimental data have been fitted assuming an appropriate sample structure. A power law correlation between the interface width and bilayer thickness has been observed for the multilayer samples, which was explained in the light of alternate roughening/smoothening of multilayers and assuming that at the interface the growth “restarts” every time.

To understand the behavior of SiO2glass under high pressure and differential stress, we conducted radial x-ray diffractionmeasurements on SiO2glass up to 60 GPa, in which x-rays irradiate the sample from a direction perpendicular to the compression axis of a uniaxial apparatus. The differential strain of SiO2glass, determined from the azimuth angle dependence of the position of the first sharp diffraction peak, was very large especially at pressures below 20 GPa and decreased with increasing pressure. After decompression, a large differential strain, equivalent to about 2 GPa in differential stress, remained in the glass at ambient conditions. We attribute this residual anisotropy to the anisotropic permanent densification, which is caused by the anisotropic change in intermediate-range structure, i.e., the anisotropic reconstruction of the network structure consisting of SiO4 tetrahedra.

This work presents a model for evaluating thermodynamic stabilization of ternary nanocrystalline alloys. It is applicable to alloy systems containing strongly segregating size-misfit solutes with a significant enthalpy of elastic strain and/or immiscible solutes with a positive mixing enthalpy. On the basis of a regular solutionmodel, the chemical and elastic strain energy contributions are incorporated into the mixing enthalpyΔHmix, and the mixing entropy ΔSmix is obtained using the ideal solution approximation. The Gibbs mixing free energyΔGmix is minimized with respect to simultaneous variations in grain size and solute segregation parameters. The Lagrange multiplier method is used to obtain numerical solutions for the minimum ΔGmix corresponding to an equilibrium grain size for given alloy compositions. The numerical solutions will serve as a guideline for choosing solutes and assessing the possibility of thermodynamic stabilization. The temperature dependence of the nanocrystalline grain size and interfacial solute excess can be evaluated for selected ternary systems. Model predictions are presented using available input data for a wide range of solvent-solute combinations. The model predictions are compared to experimental results for Cu-Zn-Zr, Fe-Cr-Zr, and Fe-Ni-Zr alloys where thermodynamic stabilization might be effective.

We describe InAsquantum dot creation in InAs/GaAsSb barrier structuresgrown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%, 13%, 15%, 16%, and 37% enclosing 2 monolayers of self-assembled InAsquantum dots.Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops, and edge segments created within the volume of the epitaxialstructures. 38% relaxation of initial elastic stress is seen for 37% Sb content, accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dotstructures particularly those with larger numbers of InAs quantum-dot layers, such as those proposed for realizing an intermediate band material.